Align protocol runtime with spec boundaries

Move demo leaf echo behavior out of the core protocol runtime, treat procedure IDs as opaque protocol fields, and return direct registered children in endpoint introspection to match the spec.

src/protocol/tree/endpoint/introspection.rs

@@ -42,6 +42,12 @@ impl ProtocolEndpoint {
             .to_vec()
         } else {
             to_bytes::<RkyvError>(&EndpointIntrospection {
+                sub_endpoints: self
+                    .children
+                    .iter()
+                    .filter(|child| child.state == super::core::ConnectionState::Registered)
+                    .filter_map(|child| child.path.get(self.path.len()).cloned())
+                    .collect(),
                 leaves: self
                     .leaves
                     .values()